Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WiFi transceivers, and other communication devices transmit and receive communication signal at various frequencies that correspond to different communication bands and at varying power levels. A typical transmitter in one of these communication devices must be capable of sending an information signal at radio frequency (RF) at a precise power level that is controlled continuously, or in small steps, over a range of approximately 90 dB. The power output of the communication device must take into account stringent operating specifications, must be substantially linear and must meet various noise and signal quality requirements.
A typical transmit system employs baseband-to-RF signal upconversion, has a power amplifier to amplify the information signal prior to transmission, and employs various impedance matching circuitry, switches, duplexers, diplexers and signal filtering circuitry. Each of these systems and elements introduces gain variations which can occur over specific components, over temperature and over frequency, thus making precise open-loop power control difficult to achieve. Power control can also be performed using a closed-loop architecture, but closed-loop power control uses additional hardware, software, calibration and battery power, has several control and timing issues and is generally problematic.
The output of the power amplifier (PA) contains the desired transmit information signal in the transmit band and also includes unwanted noise that occurs at frequencies occupied by the receive band. This unwanted noise in the receive band is created by various components in the transmitter and leaks to the input of the receiver due to finite isolation provided by elements located between the PA and the antenna. The receiver input contains the desired receive signal in the receive-band and also contains the unwanted noise that has leaked from the transmitter. Together with noise generated by the receiver, this additional noise at the receiver input results in a degraded signal-to-noise ratio (SNR) for the receiver, thus degrading its sensitivity (ability to detect weak signals). Frequently, systems employ one or more surface acoustic wave (SAW) filters between the transmitter and the PA to reduce this unwanted noise before it leaks to the receiver. However, for multiband systems, multiple SAW filters add significantly to the cost and physical size of the communication system.
For SAW-less systems, the requirements on the transmitter noise in the receive-band are much more stringent. As the transmit system gain varies from device-to-device and over frequency and temperature, the transmitter output power needs to be adjusted precisely to keep the final transmitted power at the antenna constant as required by the receiving base station. Typically, when the transmitter power is changed, its output noise also changes. However, it is required that the SNR at the transmitter output be kept constant to prevent any further de-sensitization of the receiver. This is especially important in the high-power range when the device is farthest from the basestation and the receive signal is at its weakest level.
Therefore, it would be desirable to have a transmitter system that maintains a constant SNR over a range of output power, and, a way of precisely controlling the gain of the various elements in the transmitter system so that receive band noise can be kept below a desired level and the overall power output at the antenna can be precisely controlled over a desired operating range.